1. Field of the Invention
The present invention relates to a transmitter for converting process variable-related input signals into predetermined output signals by means of multistep calculation and to a method for duplicating the transmitter.
2. Description of the Prior Art
Prior art documents related to a transmitter for converting process variable-related input signals into predetermined output signals by means of multistep calculation, such as a vibrating differential pressure transmitter, include the following:                Non-patent document: “DPharp Electronic Differential Pressure Transmitters” Yokogawa Technical Report Vol. 36 No. 1 (1992) pp. 21-28        
The standardization of safety instrumented systems (SIS) is being promoted recently for protection against bodily injury as well as environmental and equipment protection. As a result, there is a market demand for transmitters that satisfy safety integrity level (SIL) 2 (i.e., the risk reduction factor (RRF), which is the inverse number of the probability of failure on demand, is in the range of 100 to 1000).
As a rule, two or more transmitters are used for a system for which safety and reliability are required, in order to meet such a market demand.
If a sensor is highly reliable and is not required to be dual-redundant, the transmitter itself may be singular and the signal processing function thereof may be duplicated. FIG. 1 is a functional block diagram illustrating an example of a prior art differential pressure transmitter having duplicated internal signal processing functions.
Area A enclosed by a broken chain line denotes a two-wire differential pressure transmitter. The differential pressure transmitter is given an input of process variable PV (pressure, differential pressure, etc.), transmits 4-20 mA current output signal Io, and has the functions to communicate with a host computer, notify the transmitter's own failure and obtain information from the host computer.
In differential pressure transmitter A, numeral 1 denotes a vibrating pressure sensor for outputting two frequency signals-related process variable (PV). Since the structure and operating principle of this sensor are disclosed in non-patent document 1 mentioned earlier, they are not explained further.
Numerals 2 and 3 denote dual-redundant first and second counters, respectively, to which the two frequency signals are respectively input from pressure sensor 1 and counted. Numerals 4 and 5 denote dual-redundant first and second microprocessors, to which pulse signals are input from first counter 2 and second counter 3 respectively and calculated.
In first microprocessor 4, numeral 41 denotes a first calculation means for generating calculated output Do1 by performing multistep calculation and pulse width modulation on a pulse signal from first counter 2. Likewise, numeral 51 in second microprocessor 5 denotes a second calculation means for generating calculated output Do2 by performing multistep calculation and pulse width modulation on a pulse signal from second counter 3.
Numeral 6 denotes an EEPROM for retaining coefficients or the like to be referenced during corrective calculations performed by first calculation means 41. ROM7 and RAM8 are memory means used for calculations performed by first calculation means 41. Likewise, numeral 9 denotes an EEPROM for retaining coefficients or the like to be referenced during corrective calculations performed by second calculation means 51. ROM10 and RAM11 are memory means used for calculations performed by second calculation means 51.
First microprocessor 4 is a main processor. Under normal conditions, calculated output Do1 of first calculation means 41 in this processor is converted into current output signal Io and transmitted. Second microprocessor 5 is a slave microprocessor and calculated output Do2 of second calculation means 51 in this processor functions only for the purpose of checking agreement with calculated output Do1 of first microprocessor 4.
In first microprocessor 4, numeral 42 denotes a comparator and calculated output Do1 of first calculation means 41 and calculated output Do2 of second calculation means 51 are input to comparator 42. Then, agreement between outputs Do1 and Do2 are checked under predetermined conditions. If any discrepancy is found between these two outputs, alarm command AL is output.
Numeral 43 denotes an output selector for selecting calculated output Do1 of first calculation means 41 under normal conditions. Upon receipt of alarm command AL from comparator 42, output selector 43 selects burn-out signal Da from alarm signal generator 44 and inputs the signal to output means 12. Numeral 13 denotes an indicator for processing information provided by first microprocessor 4.
Under the normal conditions in which calculated output Do1 of first calculation means 41 is input through output selector 43, output means 12 converts digital calculated output Do1 into an analog value, generates current output signal Io having a 4-20 mA span, and transmits the signal to external transmission line 14.
Under the abnormal conditions in which burn-out signal Da is input from alarm signal generator 44 through output selector 43, output means 12 converts digital burn-out signal Da into an analog value, generates a 3.2 mA or 21.6 mA burn-out current output signal and transmits the signal to external transmission line 14.
Numeral 15 denotes an external DC power supply inserted in transmission line 14, numeral 16 denotes a maintenance-purpose portable communicator connected to transmission line 14, and numeral 17 denotes a host computer also connected to transmission line 14. Numeral 18 denotes a communication interface connected to output means 12. Communication interface 18 communicates with first microprocessor 4, informs host computer 17 of the occurrence of failure using a digital signal superimposed on transmission line 14, and obtains various types of information from the host computer.
Next, the details of multistep calculation performed at first calculation means 41 and second calculation means 51 will be explained by taking first calculation means 41 as a representative example. First, sensor frequencies fc and fr are calculated from pulse signals provided by first counter 2.
In first-step calculation 41a, differential pressure signal X is calculated according to the following equation using calculated fc and fr and constants A, B and C representing sensor characteristics.X=A·fc2+B·fr2+C  (Eq. 1-1)
In second-step calculation 41b, temperature- and static pressure-corrected differential pressure dpcomp is calculated as a nth-order polynomial of X according to the following equation, by using the calculated value of X and temperature- and static pressure-dependent dynamic correction factor ki stored in EEPROM 6.
                    dpcomp        =                              ∑                          i              =              0                        m                    ⁢                                          ⁢                                    k              t                        ·                          X              i                                                          (                              Eq            .                                                  ⁢            1                    ⁢                      -                    ⁢          2                )            
In third-step calculation 41c, differential pressure dpscaled having been scaled to user-specified ranges urv (100%) and lrv (0%) is calculated for the calculated value of dpcomp, according to the following equation.
                    dpscaled        =                              dpcomp            -            lrv                                urv            -            lrv                                              (                              Eq            .                                                  ⁢            1                    ⁢                      -                    ⁢          3                )            
In fourth-step calculation 41d, digital signal pwm to be pulse-modulated is calculated according to the following polynomial, using the calculated value of dpscaled and temperature-dependent dynamic correction factor Ci stored in EEPROM 6:
                    pwm        =                              ∑                          i              =              0                        n                    ⁢                                          ⁢                                    c              i                        ·                          dpscaled                                                                                ⁢                i                                                                        (                              Eq            .                                                  ⁢            1                    ⁢                      -                    ⁢          4                ⁢                                  )            
The value of digital signal pwm calculated through the four steps discussed above is input to comparator 42 as calculated output Do1 of the first microprocessor, as well as to output means 12 through output selector 43, and converted into 4-20 mA current output signal Io.
Calculations based on a plurality of calculation steps 51a to 51d executed by second calculation means 51 in second microprocessor 5, to which pulse signals are input from second counter 3, are identical with calculations based on a plurality of calculation steps 41a to 41d executed by first calculation means 41 in the first microprocessor discussed above. Calculated output Do2 is introduced to comparator 42 and compared with Do1.
Comparator 42 compares calculated outputs Do1 and Do2. If the outputs disagree beyond the predetermined allowable conditions, the comparator judges the case as a transmitter failure, outputs alarm command AL, causes output selector 43 to switch to alarm signal generator 44, causes current output signal Io to go into a burnout state, and informs host computer 17 of the transmitter failure.
Numeral 45 denotes a self-diagnosis means for performing a fault diagnosis on pressure sensor 1 (frequency failure or the cessation of vibration in the case of vibrating sensors) or checking the microprocessor itself for a possible operational failure if the signal of counter 2 or 3 stops or if the transmitter output is lost or does not change for a specific period of time. If self-diagnosis means 45 detects any failure, it transmits signal Ds to output selector 43 to cause current output signal Io to go into a burnout state.
As a rule, two or more transmitters need to be used for a system for which safety is required, thus involving high instrumentation costs. If the sensor is highly reliable and therefore dual-redundancy is not required, the system may be configured so that the transmitter itself is singular and the internal signal processing function is duplicated, as illustrated in FIG. 1. According to this system configuration, it is possible to reduce the abovementioned costs, compared with the case when hardware is completely duplicated.
However, since second counter 3, second microprocessor 5, and memory means 9 to 11 are added as hardware components even in the transmitter configuration illustrated in FIG. 1, the cost problem is not completely solved. In addition, an increase in the number of hardware components constitutes an obstacle to downsizing transmitters.